The use of windmills to generate electrical power is well known. Not only windmills but other kinds of wind-driven devices have been used or proposed for generating electrical power. Thousands of small wind-driven generators were in operation in rural areas in the 1930's and 1940's to supply power to farmers before power lines were built. Such systems were of relatively simple construction and included a small generator directly driven by a two-blade propeller of conventional construction having a span of a few feet. Because of widely varying wind velocities, alternating currents would have been of varying frequencies, so the generator was connected to charge a bank of batteries and the power used as direct current. Where it is desired to supply and use alternating current in large quantities, the system becomes much more complex. Since a generator inherently supplies alternating current at a frequency varying with its rotational speed, a number of schemes have been proposed for controlling the feathering of propellers to keep their speed relatively constant, or within a narrow range of speeds such that other means may be used to drive the generator at a constant speed.
Where such windmills are used to generate power in commercial quantities to be supplied to power lines, economic considerations point to efficiencies in building very large windmills of diameters in excess of 40 meters. The large cost of such systems puts a considerable premium on efficiency of the installation. Used in this sense, efficiency refers to the ability of the windmill blade to extract the maximum amount of energy for the range of wind velocities experienced. To do this successfully, the windmill should be structurally capable of withstanding high wind velocities such as in the order of 70-80 kilometers per hour without requiring total feathering and shutdown of the system. It should also include feathering means for protecting the windmill from wind velocities in excess of values determined to be safe for the equipment.
One of the problem areas with windmills having a horizontal shaft is that since the wind blows substantially perpendicular to the direction of blade rotation and impinges on the blades from immediately outboard of the hub to the blade tips, there is a very great difference in blade velocity from one radial position to another. It has long since been determined that windmill blades are, in general, most efficient when constructed with an airfoil-like cross-section. With the differences in velocities along the blade, however, it is almost inevitable that one part of the blade or another will be in a condition of stall. Stalling frequently begins near the root of the blade even when the blade is rotating at relatively high speed. The stall region may then tend to migrate along the blade radially until the blade no longer has sufficient power to carry the load, and the entire blade will then stall. It is known in some designs to put twist in the blade from the hub to to the tip avoid or minimize stalling of the blade. There is, therefore, a need for a windmill blade configuration which is as efficient as possible, which minimizes stalling, and which is rugged and durable enough to operate and thereby generate power at substantial wind velocities and resulting high rotational speeds.